The present invention relates in general to digital telecommunication systems, and is particularly directed to a mechanism for augmenting the T1 voice channel-carrying capacity of a T1 digital communication link, by operating the link as a higher bandwidth signaling protocol link, such as an E1 link, and multiplexing onto the link, in addition to an entire T1 frame of twenty-four DS0 channels, a plurality of (up to six) additional DS0 (data) channels without converting the signaling properties of the T1 channels into E1 protocol.
With the ongoing demand for increased bandwidth capacity, digital network service providers are continuously seeking ways to extract more performance from their existing communication network infrastructures. In particular, there is a continuing demand for more bandwidth, while voice transport demand has not diminished, so that both voice and data are typically transported over the same channelized path (e.g., local loop). In the United States, the existing digital communication infrastructure employs basic rate (T1) channelized time division multiplexed (TDM) digital communication protocol, which is defined as twenty-four DS0 (e.g., voice) channels, each providing 64 Kbps worth of bandwidth, for a total or cumulative T1 bandwidth capacity of 1.536 Mbps. As shown in the time slot/channel diagram of FIG. 1, a respective channelized T1 frame contains twenty-four, eight-bit bytes or time slots TS1-TS24, plus a frame sync bit, for a total of 193 bits per frame (which corresponds to an overall clock rate of 1.544 Mhz). In order to convey signaling control information for a respective DS0 voice channel TS1, selected least significant bits, termed A/B (C/D) bits, are periodically xe2x80x98robbedxe2x80x99 and used as signaling bits. Voice signals are encoded using mu-law coding.
Other, non-domestic networks, on the other hand, such as those installed in Europe, and Central and South America, employ E1 rate channelized TDM digital communication protocol, which has a higher overall clock rate (2.048 MHz) and available information transport bandwidth capacity (1.920 Mbps). As shown in the time slot/channel diagram of FIG. 2, a respective E1 frame contains thirty-two, eight-bit bytes/time slots TS0-TS31. Of these thirty-two time channels/time slots, one channelxe2x80x94channel sixteen TS16xe2x80x94is used for signalling information; another channelxe2x80x94channel TS0xe2x80x94serves as a frame synchronization channel. Voice is encoded using A-law coding. The remaining thirty channels (TS1-TS15 and TS17-TS31) provide a total available information transport bandwidth capacity of 1.920 Mbps, which exceeds that of a standard T1 link by 384 Kbps, or six DS0 channels (and happens to be the number of T1 channels required for achieving high efficiency data transport).
Because of differences in their framing structures and the fact that their signaling coding schemes and voice encoding (mu-law vs. A-law) are mutually incompatible, T1 networks and E1 networks cannot be readily interchanged for one another. Instead, the two are customarily interfaced by means of a relatively complex network converter (a T1-E1 converter when going from a T1 system (e.g., in the United States) to an E1 system (e.g., in Mexico), and an E1-T1 converter when going from an E1 to an T1 system). Unfortunately, current T1-E1 network converters simply map T1 protocol into E1 protocol, without taking advantage of the additional unused portion of the E1 bandwidth. Although, in such instances, the transport efficiency of the E1 link is less than optimum, it at least provides delivery of the T1 channels to the far end of the E1 link.
In accordance with the present invention, advantage is taken of additional bandwidth available in an elevated transport rate-based digital communication link to transport both an entire T1 frame of twenty-four DS0 voice/data channels, and a plurality of (up to six) additional DS0 data channels without converting the signaling properties or voice coding of the T1 frame into the higher transport rate protocol. As a non-limiting example, the elevated transport rate-based digital communication link may be operated at the augmented 2.048 MHz clock rate of an E1 link. This elevated clocking rate of the link is effective to increase its DS0 transport capacity from twenty-four channels (associated with a standard T1 data rate of 1.536 Mbps) to thirty channels (associated with a standard E1 data rate of 1.920 Mbps).
Rather than change the protocol and coding of the T1 voice/data channels, these channels and additional DS0 channels are multiplexed into the time slots/channels of the E1 framing structure. For the case of non-primary T1 voice channels, robbed signaling bits are inserted into the E1 signaling channel. Each end of the link is coupled to its own T1/E1 multiplexer/demultiplexer (mux/demux), that is configured of an intercoupled arrangement of conventional digital data communication T1 and E1 framers, plus a standard digital access and cross-connect system (DACS). The E1 framer is coupled to the communication link, and to a robbed bit signaling port of a voice channel T1 framer through which up to twenty-four DS0 voice/data channels are interfaced with mux/demux. A data channel T1 framer is coupled to interface a limited number of additional (e.g., up to six) DS0 data channels with the mux/demux. The DACS is coupled to each of the T1 framers and interfaces DS0 channels with the E1 framer.
In a typical non-primary rate channelized application, the voice/data channel T1 framing unit interfaces the twenty-four T1 voice channels it receives with the DACS, while the data channel T1 framer interfaces the (six) data channels with the DACS. The DACS couples the total of these thirty DS0s to the E1 framer for insertion into thirty time slots of the outgoing E1 frame. Frame sync bits are carried by E1 channel TS0, while the robbed signaling bits are applied directly from the T1 framer to the E1 framer using channel associated signaling for insertion into the E1 signaling channel TS16. At the receive end of the link, the far end E1 framer couples the contents of the thirty DS0 information (voice and data) channels to the DACS for distribution to associated voice channel and data channel T1 framers. It also extracts the robbed signaling bit contents of the signaling channel to the voice channel T1 framer for insertion into outgoing T1 voice channels.
For primary rate ISDN, since their are no robbed signaling bits, the signaling channel of the E1 frame becomes available for any ISDN channel. As a result, the control xe2x80x98Dxe2x80x99 channel of the T1 primary rate ISDN frame may be transported in any of the E1 frame""s thirty-one available time slots. Demultiplexing at the receive end of the link is essentially the same as described above for the non-primary rate case, except that there is no robbed bit signaling transfer between the E1 framer and the voice channel T1 framer. Instead, all thirty-one channels are coupled directly to the DACS, which transfers the twenty-four primary rate ISDN channels to the voice channel T1 framer and the remaining six DS0s to the data channel T1 framer. Once again, the voice coding would not change (xcexc-law to A-law).